Alignment solutions on FBEOL layers using ASML scanners

Wafers at FBEOL layers traditionally have higher stress and larger alignment signal variability. ASML's ATHENA sensor based scanners, commonly used to expose FBEOL layers, have large spot size (similar to 700um). Hence ATHENA captures the signal from larger area compared to the alignment marks which are typically similar to 40um wide. This results in higher noise in the alignment signal and if the surrounding areas contain periodic product structures, they interfere with the alignment signal causing either alignment rejects or in some cases-misalignment. SMASH alignment sensors with smaller spot size (similar to 40um) and two additional probe lasers have been used to improve alignment quality and hence reduce mark/wafer rejects. However, due to the process variability, alignment issues still persist. For example, the aluminum grain size, alignment mark trench deposition uniformity, alignment mark asymmetry and variation in stack thicknesses all contribute to the alignment signal variability even within a single wafer. Here, a solution using SMASH sensor that involves designing new alignment marks to ensure conformal coating is proposed. Also new techniques and controls during coarse wafer alignment (COWA) and fine wafer alignment (FIWA) including extra controls over wafer shape parameters, longer scan lengths on alignment marks and weighted light source between Far Infra-Red laser (FIR) and Near Infra-Red (NIR) for alignment are presented. All the above mentioned techniques, when implemented, have reduced the wafer alignment reject rate from around 36% to less than 0.1%. Future work includes mark validation based on the signal response from the various laser colors. Finally, process monitoring using alignment parameters is explored.